Evaluate candidate vaccines (viral or bacterial) in infants under one year of age and young children. Conduct, depending on the vaccine to be tested, Phase I studies in semi-closed or carefully monitored open populations of infantsand children. Determine, depending on the vaccine to be tested, the safety, infectivity, degree of virulence, reactogenicity, transmissibility, genetic stability, and immunogenicity. Follow vaccinees for two to three years to document any potential hazards of immunication, duration of antibody, and resistance in the event of natural challenge within the community. Initiate and maintain epidemiologic and laboratory surveillance for infection of the major viral pathogens of infancy and childhood in the population proposed for this study. Conduct appropriate assays for viral antigens and antibodies. Evaluate antiviral substance for prophylaxis and therapy of viral infections.